The use of heliostats in the field of concentrating solar power (CSP) is well established in the prior art. A typical CSP system includes at least one centralized tower and a plurality of heliostats corresponding to each centralized tower. The tower is centralized in the sense that the tower serves as the focal point onto which a corresponding plurality of heliostats collectively redirect and concentrate sunlight onto a target (also referred to as a focus or a receiver) associated with the tower. The concentration of sunlight at the tower receiver is therefore directly related to the number of heliostats associated with the tower up to certain fundamental limits. This approach concentrates solar energy to very high levels, e.g., on the order of 1000× or more if desired. In practical application, many systems concentrate sunlight in a range from 50× to 5000×. The high concentration of solar energy is converted by the tower into other useful forms of energy. One mode of practice converts the concentrated solar energy into heat to be used either directly or indirectly, such as by generating steam, to power electrical generators, industrial equipment, or the like. In other modes of practice, the concentrated solar energy is converted directly into electricity through the use of any number of photovoltaic devices, also referred to as solar cells.
Heliostats generally include a mirror or other suitable optical device to redirect sunlight, support structure to hold the mirror and to allow the mirror to be articulated, and actuators such as motors to effect the articulation. At a minimum, heliostats must provide two degrees of rotational freedom in order to redirect sunlight onto a fixed tower focus point. Heliostat mirrors may be planar, but could possibly have more complex shapes. Heliostat articulation can follow an azimuth/elevation scheme by which the mirror rotates about an axis perpendicular to the earth's surface for the azimuth and then rotates about an elevation axis that is parallel to the earth's surface. The elevation axis is coupled to the azimuth rotation such that the direction of the elevation is a function of the azimuth angle. Alternatively, heliostats can articulate using a tip/tilt scheme in which the mirror rotates about a fixed tip axis that is parallel to the earth's surface and a further tilt axis. The tip axis often is orthogonal to the tilt axis but its axis of rotation tips as a function of the tip axis rotation. The tilt axis is parallel to the earth's surface when the heliostat mirror normal vector is parallel to the normal vector of the earth's surface. Other schemes, such as polar tracking and many others, are also possible; the present invention is applicable to any of these schemes.
Heliostats are pointed so that the reflected sunlight impinges on the central tower receiver, which often is fixed in space relative to the heliostat. Because the sun moves relative to the heliostat site during the day, the heliostat reflectors must track the sun appropriately to keep the reflected light aimed at the receiver as the sun moves.
FIG. 1 schematically illustrates a typical CSP system 403. CSP system 403 has tower 405 with focus region 407 and a plurality of corresponding heliostats 409 (only one of which is shown for purposes of illustration) that aim reflected sunlight at region 407. Sunlight represented by vector 411 reflects off the heliostat mirror 413 oriented with surface normal represented by vector 415. Mirror 413 is accurately aimed so that reflected sunlight according to vector 417 is aimed at focus 407 generally along heliostat focus vector 419, which is the line of sight from the heliostat mirror 413 and the tower focus 407. If mirror 413 were to be aimed improperly so that vector 417 is not aimed at focus 407, these two vectors would diverge. Consequently, the reflected light 417 impinges on the tower focus 407. For such conditions to be realized, the laws of reflection require that the angle formed between the sunlight vector 411 and mirror normal 415 must be equal to the angle formed between vector 419 and mirror normal 415. Further, all three vectors 411, 415, and 419 must lie on the same plane. It can be shown using vector algebra that given a sunlight vector 411 and focus vector 419, there is a unique solution for mirror normal 415 that is simply the normalized average of vectors 411 and 419.
Many control strategies use open loop control, closed loop control, or combinations of these. Many heliostat control systems employ open loop algorithms based on system geometry and sun position calculators in order to determine the sun and heliostat-focus vectors as a function of time. These calculations result in azimuth/elevation or tip/tilt commands to each heliostat device. Such control systems generally assume that the locations of the heliostats are static and well defined and/or otherwise rely on periodic calibration maintenance to correct for settling and other lifetime induced drifts and offsets. Open loop solutions are advantageous in that they do not require any feedback sensors to detect how well each heliostat is pointed. These systems simply tell every heliostat how to point and assume that the heliostats point correctly. A major drawback is that open loop systems demand components made with high precision if accuracy is to be realized. Incorporating precision into the system components is very expensive. Additionally, it can be cost prohibitive to perform the precise surveying needed to perform open loop calculations with sufficient accuracy. The expense of precision and surveying escalates as the number of heliostats in a heliostat field increases. Consequently, systems that rely only on open loop control tend to be too expensive.
Closed loop heliostat control relies on feedback from one or more sensors capable of measuring differences, or errors, between the desired condition and an actual condition. These errors are then processed into compensation signals to heliostat actuators to articulate the mirrors so that reflected sunlight impinges on the tower focus. Closed loop pointing has an advantage that it does not require precise components or installation or knowledge of the system geometry. The system also can be made less sensitive to lifetime drifts. Less demand for precision means that these systems are much less expensive than systems that rely solely on open loop control. Closed loop systems offer the potential to use control software rather than predominantly precision, and control is much less expensive to implement than precision.
A difficulty in applying closed loop pointing methods on CSP systems results from the pointing condition requiring the bisection of two vectors rather than alignment to a single vector. That is, as show in FIG. 1, during normal operation, the heliostat mirror 413 itself doesn't point at anything in particular—rather, it must point in a direction 415 in between the sun 411 and the target 407, and the point moves with time as the sun moves. Nominally, there is nothing in that direction but empty sky, so there is nothing for a traditional closed loop tracking system to point the mirror at.
The ideal closed loop heliostat tracking system should sense the difference between the reflected sunlight vector 417 and the line of sight vector 419, and endeavor to control that difference to zero. Thus, CSP and concentrated photovoltaic (CPV) system designers have contemplated that an ideal location for a feedback sensor would be to place the sensor in the path of the reflected beam, such as at the tower focus 407. Unfortunately, this is not feasible because no practical sensor could withstand the extreme temperatures or the UV dosage that result from highly concentrated sunlight. This poses a significant technical challenge of how to track and correct the aim of a beam if the beam cannot be tracked.
Other schemes are possible, albeit less desirable. For example, one prior art system (http://www.heliostatus/howitworks.htm) discloses a sensor that controls sunlight vector 417 to be aligned with a third vector, the axis of a sensor near the heliostat. During installation of the system, the sensor is aligned with the line of sight vector 419. The accuracy of the system is thus dependent on the accuracy of this alignment, and on the alignment remaining unchanged. In large CSP systems, however, this may be insufficient for several reasons; for example, the tower 405 may sway in the wind or experience thermal expansion or contraction. Cost may also be an issue, since each heliostat requires a separate sensor.
A second type of “closed loop” heliostat system that is common in the prior art is a system that senses the orientation of the heliostat axes with respect to the heliostat base. That is, referring to FIG. 3, such a system may provide encoders that measure the rotations of axes 29 and 33. The control system then provides corrections to any detected errors in the orientation of these axes. This type of system mitigates errors in the gear train of the heliostat or errors, but it does not sense the sunlight vector 417 at all, so it is susceptible to any unseen errors in this vector, and it is blind to any errors in the alignment of sunlight vector 417 to line of sight 419. This system thus likewise may be sensitive to motions of the tower and long-term drifts. Practical systems tend to include elaborate calibration schemes to deal with these issues. Cost also is impacted, since encoders are needed for each axis of each heliostat.
Consequently, there remains a strong need for techniques that would allow closed loop pointing to be feasible.